The invention relates to rotary drill bits for use in drilling holes in subsurface formations and particularly to drag type drill bits of the kind comprising a bit body having a leading face and a gauge region, a plurality of blades formed on the leading face of the bit and extending outwardly away from the axis of the bit towards the gauge region so as to define between the blades a plurality of fluid channels leading to junk slots in the gauge region, a plurality of cutting elements mounted along each blade, and a plurality of nozzles in the leading face of the bit for supplying drilling fluid to the channels for cleaning and cooling the cutting elements.
One of the major problems in designing a drill bit of this type lies in positioning and orientating the nozzles so as to provide the most effective flow pattern of drilling fluid along the channels and across the cutting elements to obtain the optimum cleaning and cooling effect. The flow of drilling fluid emerging from the nozzles and impinging on the surface of the formation being drilled also serves to cool the formation.
Normally the nozzles are located in the area around the central axis of rotation of the bit so that substantially all of the drilling fluid emerging from the nozzles flows outwardly along the channels with which each nozzle communicates, so as to wash over the cutting elements facing into the channels. Arrangements are known in which these nozzles are slightly angled with respect to the surface of the bit body so that the drilling fluid emerges from the nozzle in a direction having a component along a particular channel. However, where a nozzle is feeding two or more channels it may be undesirable to direct the flow along a particular channel.
Another requirement for the flow of drilling fluid is effectively to remove and clear from the channels the cuttings which are removed from the formation by the cutting element as drilling proceeds. In soft and sticky formations there may be a tendency for such cuttings to accumulate or "ball" in the channels and it is necessary that the flow of drilling fluid should be capable of preventing or clearing such accumulations since otherwise the cutting elements facing into a blocked channel may become ineffective for drilling.
Many different designs of drill bit, and arrangements of nozzles, have been proposed in an endeavour to meet these requirements. In International Specification No. WO84/01186 there is shown an arrangement in which nozzles are provided near the outer periphery of the drill bit instead of near the centre. In such arrangement there are not provided junk slots directly adjacent the peripheral nozzles so that the drilling fluid emerging from the nozzles cannot flow directly up the annulus between the drill String and the bore hole. Instead, the blades and channels are so arranged that drilling fluid emerging from the peripheral nozzles flows first inwardly along one channel towards the centre of the drill bit and then flows outwardly again along a further channel leading to a junk slot.
In European Specification No. 0171915 arrangements are shown in which conventional nozzles in near the axis of the drill bit are supplemented by peripheral nozzles nearer the gauge region which are so arranged in relation to the adjacent blades that drilling fluid from the peripheral nozzles flows substantially tangentially in a peripheral direction around the outer periphery of the drill bit so as to cool and clean cutting elements which lie in the peripheral flow path between each peripheral nozzle and the nearest junk slot.
The present invention provides an improved arrangement where nozzles near the central axis of the drill bit are supplemented by peripheral nozzles nearer the gauge region, the nozzles being so located and orientated as to provide an improved effect when compared with the prior art arrangements.